Stack overflow or compiler hangs on opaque types with cyclic references

[panacekcz]

Test code 1:

object O1 {
  opaque type X = Array[X]
}

Output:

java.lang.StackOverflowError while compiling opaque-cycle-1.scala
Exception in thread "main" java.lang.StackOverflowError
        at dotty.tools.dotc.core.Types$Type.dealias1(Types.scala:1073)
        at dotty.tools.dotc.core.Types$Type.dealias(Types.scala:1091)
        at dotty.tools.dotc.core.Definitions$ArrayOf$.unapply(Definitions.scala:926)
        at dotty.tools.dotc.core.TypeErasure.eraseArray(TypeErasure.scala:480)
        at dotty.tools.dotc.core.TypeErasure.dotty$tools$dotc$core$TypeErasure$$apply(TypeErasure.scala:422)
        at dotty.tools.dotc.core.TypeErasure.eraseArray(TypeErasure.scala:483)
        at dotty.tools.dotc.core.TypeErasure.dotty$tools$dotc$core$TypeErasure$$apply(TypeErasure.scala:422)
        at dotty.tools.dotc.core.TypeErasure.eraseArray(TypeErasure.scala:483)
        (repeats)

Test code 2:

object O2 {
  opaque type X = X
}

Compiler hangs.

Test code 3:

object O3 {
  opaque type R[X] = T[X]
  opaque type T[X] = R[X]
}

Output:

java.lang.StackOverflowError while compiling opaque-cycle-3.scala
Exception in thread "main" java.lang.StackOverflowError
        at dotty.tools.dotc.core.Substituters.substParams(Substituters.scala:154)
        at dotty.tools.dotc.core.Substituters.substParams$(Substituters.scala:149)
        at dotty.tools.dotc.core.Contexts$Context.substParams(Contexts.scala:71)
        at dotty.tools.dotc.core.Substituters$SubstParamsMap.apply(Substituters.scala:196)
        at dotty.tools.dotc.core.Types$TypeMap.mapOver(Types.scala:4408)
        at dotty.tools.dotc.core.Substituters.substParams(Substituters.scala:160)
        at dotty.tools.dotc.core.Substituters.substParams$(Substituters.scala:149)
        at dotty.tools.dotc.core.Contexts$Context.substParams(Contexts.scala:71)
        at dotty.tools.dotc.core.Types$Type.substParams(Types.scala:1443)
        at dotty.tools.dotc.core.Types$LambdaType.instantiate(Types.scala:2909)
        at dotty.tools.dotc.core.Types$LambdaType.instantiate$(Types.scala:2908)
        at dotty.tools.dotc.core.Types$HKLambda.instantiate(Types.scala:2938)
        at dotty.tools.dotc.core.TypeApplications$.tryReduce$1(TypeApplications.scala:375)
        at dotty.tools.dotc.core.TypeApplications$.$anonfun$appliedTo$extension0$1(TypeApplications.scala:392)
        at dotty.tools.dotc.util.Stats$.track(Stats.scala:37)
        at dotty.tools.dotc.core.TypeApplications$.appliedTo$extension0(TypeApplications.scala:354)
        at dotty.tools.dotc.core.Types$AppliedType.derivedAppliedType(Types.scala:3497)
        at dotty.tools.dotc.core.Types$TypeMap.derivedAppliedType(Types.scala:4350)
        at dotty.tools.dotc.core.Types$TypeMap.mapOver(Types.scala:4408)
        at dotty.tools.dotc.core.Substituters.substParams(Substituters.scala:160)
        at dotty.tools.dotc.core.Substituters.substParams$(Substituters.scala:149)
        at dotty.tools.dotc.core.Contexts$Context.substParams(Contexts.scala:71)
        at dotty.tools.dotc.core.Types$Type.substParams(Types.scala:1443)
        at dotty.tools.dotc.core.Types$LambdaType.instantiate(Types.scala:2909)
        at dotty.tools.dotc.core.Types$LambdaType.instantiate$(Types.scala:2908)
        at dotty.tools.dotc.core.Types$HKLambda.instantiate(Types.scala:2938)
        at dotty.tools.dotc.core.TypeApplications$.tryReduce$1(TypeApplications.scala:375)
        at dotty.tools.dotc.core.TypeApplications$.$anonfun$appliedTo$extension0$1(TypeApplications.scala:392)
        at dotty.tools.dotc.util.Stats$.track(Stats.scala:37)
        at dotty.tools.dotc.core.TypeApplications$.appliedTo$extension0(TypeApplications.scala:354)
        at dotty.tools.dotc.core.Types$AppliedType.derivedAppliedType(Types.scala:3497)
        at dotty.tools.dotc.core.Types$TypeMap.derivedAppliedType(Types.scala:4350)
        (repeats)